1. Field of the Invention
This invention relates to rotary fluid control valves of the butterfly type wherein a wafer or disc is mounted for rotation between an open position, in which the disc lies substantially parallel to the axis of the fluid flow channel through the valve, and the closed position in which the disc lies perpendicular to this axis. The disc, in its closed position, cooperates with an annular seat circumscribing the fluid flow channel to shut off fluid flow through the channel. The annular seat is held in position by being clamped in a recess formed between complementary surfaces of a portion of the valve body and a valve seat insert. The insert may be bolted to the valve body, or more commonly, the insert is fixed in position when the valve is bolted between the pipe flanges formed on the pipes that convey fluid to and from the valve. An example of this type of valve is described in U.S. Pat. No. 4,331,319, which is herein incorporated by reference.
2. Description of the Prior Art
With a butterfly valve of the type just described, it is desirable to provide features that will optimize the sealing effectiveness of the valve, yet minimize disadvantages apparent in prior art butterfly valves. For example, it is advantageous for a butterfly valve to provide a tight seal regardless of the direction of fluid flow, and continue to do so despite continuous reversal of pressure differentials across the valve. Further, pressure up to and including the full valve pressure rating should be appliable from either direction without leaking across the valve.
Butterfly valves are frequently subjected to a wide range of temperatures, both from the ambient environment in which they operate and from the flowing media being controlled. Further, the temperature will frequently cycle over a relatively wide range and it is advantageous for a butterfly valve to withstand these temperature variations without adverse effects on its sealing capabilities.
The temperature variations described above, as well as bi-directional fluid pressure, can cause distortions of the valve seat which will result in improper sealing between the seat and the butterfly valve disc. In the prior art, it was common to form the valve seat of an elastomeric material, or to provide the valve seat with an elastomeric insert at the sealing surface. The elastomeric material had the advantage that if thermal or fluid pressure stresses caused distortions of the valve seat which would tend to inhibit complete sealing the valve disc and the seat, the elastomeric material at the sealing surface could distort, under the closing torque of the valve seat, so as to deform into the shape of the peripheral surface of the disc and establish a perfect seal.
A further advantage of elastomeric materials for the valve seat, or for sealing inserts of the valve seats, was the tendency of metal sealing surfaces to leak after a few closure cycles as a result of scratches occurring in valve seats having metal seals. That is, unless hardened surfaces were provided for these metal valve seats, impurities in the flowing fluid had a tendency to abrade and scratch the metal valve seats after a few closing cycles. The pressurized fluid could leak past the metal valve seat sealing surface through these scratches.
However, the use of elastomeric materials as the sealing surfaces for valve seats has not been found to be completely satisfactory either. First, although elastomeric valve seat sealing surfaces do not leak as readily as metal valve seat sealing surfaces upon the occurrence of abrasion or wear, the elastomeric valve seat sealing surfaces are more susceptible to softening and failure at high temperatures than metal valve seat sealing surfaces and so must be replaced more often. Metal seats, on the other hand, are not subject to such softening. Second, where flammable fluids are being conveyed, the risk of fire exists and elastomeric valve seats, or valve seat sealing surfaces, may be quickly destroyed under such circumstances.
It is therefore advantageous for a butterfly valve to have a seat that can accommodate thermal and pressure stresses, that will not leak after a few cycles of use, that has a long life and that will be effective at high and low temperatures.